Lubricants are often used in demanding environments where degradation of the lubricant base stock can lead to catastrophic results, but where such degradation is accelerated by high temperatures, extreme wear conditions, acidic or other corrosive conditions, etc. For example, automobile engines require periodic oil changes to replace degraded engine lubricant in order to protect against undue wear and engine damage.
Alkylated diaryl amines, such as alkylated diphenylamines (ADPAs), are well known antioxidants widely used to prevent degradation and maintain the performance of engine oils found in gasoline and diesel engines for cars and trucks, as well as a variety of industrial lubricants and lubricants for marine engines, etc. Motor oil drain intervals have been significantly extended in recent years through the use of antioxidant formulations, coupled with improved base-stocks. Extending drain intervals conserves oil, reduces used oil and filter waste, and helps reduce the illegal disposal of used oil.
When selecting an ADPA or other diaryl amine antioxidant for use a number of performance, safety and environmental concerns must be addressed. For example, diphenylamine itself has good antioxidant activity but is known to be a sensitizer and its presence is typically kept to a minimum, e.g., less than 1% of any ADPA antioxidant. Diphenylamines substituted with hydrocarbyl groups are more soluble in lubricating oil and the higher molecular weight reduces volatility. Increased alkylation also helps to solubilize polar materials formed from oligomerization of spent oxidized amines, which reduces deposits, sludge and varnish. On the other hand, the antioxidant activity of ADPAs is dependent on the concentration of nitrogen provided and is thus inversely proportional to molecular weight and so excessive alkylation or very large alkyl groups should be avoided.
Also, alkylation of the aromatic ring at the ortho-position relative to the nitrogen often diminishes the activity of the amino group, making para-substituted ADPAs more valuable as anti-oxidants. For example, tri-alkylation of DPA generally provides products wherein at least one ring is ortho, para- substituted, making many tri-alkylated diphenyl amines undesirable. Diphenyl amines that are mono-alkylated or di-alkylated by moderately sized alkyl groups at the para position relative to nitrogen therefore typically have performance advantages over many other ADPAs in many lubricant formulations.
When formulating lubricants, liquid components, i.e., components that are liquid at room temperature, e.g., approximately 20 to 25° C., are easier to handle than solid components in the blending process and are also less likely to cause pour-point, gelling, or filter-clogging problems by crystallizing out of an oil. Many commercial ADPAs are mixtures that tend to be liquid when containing certain ratios of mono-, di-, and tri-alkylated diphenylamines. As discussed above, it is anticipated that eliminating tri-alkylated materials from such mixtures could improve performance, but many processes for alkylating diphenyl amines are not selective enough to provide major amounts of di-alkylated products without producing measurable amount of tri-alkylated diphenyl amines.
U.S. Pat. No. 6,204,412 discloses that the formation of symmetrically disubstituted diphenylamines can often increase the melting point of the alkylated diphenylamine composition and can lead to a solid alkylated diphenylamine composition rather than a liquid. For example, a composition with 25 wt % or more dioctyldiphenylamine, obtained e.g., by reaction between diphenylamine and diisobutylene, is typically solid at room temperature.
U.S. Pat. No. 6,315,925 provides a liquid mixture of nonylated diphenylamines consisting essentially of, as measured by gas chromatography, a) from 68% to 78% by area dinonyldiphenylamine, b) from 20 to 30% by area nonyldiphenylamine, c) from 1.0 to 3.5% by area trinonyldiphenylamine, and d) from 0.1 to 1.0% by area diphenylamine, and a process for the preparation thereof by using acid catalysts in small quantities.
U.S. Pat. No. 6,355,839 discloses a process comprising alkylating diphenylamine with a highly reactive polyisobutylene (HR PIB) having an average molecular weight of 120 to 600 wherein the polyisobutylene contains at least 25% methylvinylidene isomer. The HR PIB is a distillate byproduct from a commercial reaction, contains mostly C8H16 to C28H56 oligomers and is typically sold as a waste product. Diisobutylene, C8H16, is present in amounts of 50% or less meaning that at least half and typically a majority of the HR PIB comprises oligomers having 12 or more carbon atoms that generate products having for moderate to large alky substituents. Although it is a relatively inexpensive starting material, the makeup of the HR PIB, and therefore the makeup of the alkylated diphenyl amine, will vary and depend on the fraction of byproduct that is collected during distillation. Similarly, U.S. Published Patent Application No. 2010/0173811 discloses para-alkylated diphenylamines made by alkylating diphenylamine with a propene oligomer mixture in which the oligomer present in the greatest percentage has 15-24 carbon atoms.
U.S. Pat. No. 7,847,030 discloses a process for making a diphenylamine-functionalized poly-alpha-olefin from another commercial distillate byproduct comprising mostly C8H16 to C30H60 oligomers, (average C20H40), with an average molecular weight of about 280, which contains terminal olefins in the amount of at least 25%. A preferred product is a mixture of PAO dimers mono-functionalized and di-functionalized with diphenylamine e.g.,:
wherein R1 and R2 are independently selected from the group consisting of hydrogen, C1-12 alkyl or aryl group and R3 and R4 are independently selected from the group consisting of a linear alkyl group of 8 to 30 carbons and an aryl group of 8 to 30 carbons.
Anti-oxidants are needed for lubricants that are highly effective and readily handled, e.g., liquids or soluble low melting solids, that meet personal safety and environmental standards, and which can be made consistently, safely and economically from readily obtained starting materials. It has been found that certain small to moderately sized olefins containing no more than 16 carbon atoms, many of which are commercially available or readily available from well-known alcohols, will react efficiently and selectively with diaryl amines to produce highly effective alkylated diaryl amine antioxidants.